Starting valve for variable displacement hydraulic pump



March 26, 1963 'r-. BUDZICH 3,082,593

STARTING VALVE FOR VARIABLE DISPLACEMENT HYDRAULIC PUMP Filed May- 23. 1961 2 SheetS'-Sheet 1 IN VEN TOR. Q man/s2 BUDZ/CH March 26, 1963 1-. BUDZICH 3,082,693

STARTING VALVE FOR VARIABLE DISPLACEMENT HYDRAULIC PUMP Filed May 23. 1961 2 Sheets-Sheet 2 INVEN TOR. 7405052 BUDZ/Ch 3,082,693 Patented Mar. 26, 1963 3,082,693 STARTING VALVE FOR VARIABLE DISPLACE- MENT HYDRAULIC PUMP Tadeusz Budzich, Shaker Heights, Ohio, assignor to The Weatherhead Company, Cleveland, Ohio, a corporation of Ohio Filed May 23, 1961, Ser. No. 111,946 7 Claims. (Cl. 103-39) This invention relates to variable volume hydraulic pumps and more particularly to a variable volume pump of the type disclosed in my co-pending application, Serial No. 88,142, filed February 9, 1961.

Variable volume pumps having a plurality of pumping cylinders in a cylinder block movably mounted to change the position of the inlet port relative to the piston and thereby vary the effective pumping stroke provide a number of advantages with respect to efficiency in operation, simplicity in construction and low cost of manufacture. Pumps of this general type are also disclosed in my copending application Serial No. 822,005, filed July 6, 1959. These pumps are particularly adapted to use the bearing structure disclosed in my co-pending application Serial No. 17,832, filed March 28, 1960, in which the rotation of the drive member and the pumping loads on the wobble plate are utilized to maintain the oil film between the bearing parts when the pump is running to minimize the friction in the drive mechanism without resorting to a more expensive construction using antifriction bearings. However, in pumps of this type there is a tendency for the pump outlet pressure to build up rapidly and therefore create a high pumping load on the bearings before the pump is up to speed and before the lubricating oil film has been properly formed on the hearing surfaces. To prevent this rapid initial pressure build up, it has been proposed that a pressure responsive starting valve be employed to connect the pump outlet to drain and prevent outlet pressure build up until the output volume of the pump reaches a level corresponding to the full operating speed of the pump and the lubricating oil film has been formed. One form of starting valve has been disclosed in my co-pending application Serial No. 847,- 512, filed October 20, 19597 However, pressure responsive valves tend to be unreliable in this application because conditions in the external fluid circuit may cause the valve to close too soon before full speed has been reached or the valve may not close at full speed and the pump becomes ineffective. Furthermore, such valves add to the pump structure and defeat the purpose of using plain bearings by increasing the cost of manufacture.

It is therefore among the objects of the present invention to provide in a variable displacement pump having an axially slidable cylinder block and a pressure responsive control, a novel starting valve adapted to prevent pressure build up at the pump outlet until the pump has attained suflicient speed to allow the oil film to be formed on the bearing surfaces.

It is another object of this invention to provide a starting valve for variable displacement pump which is incorporated into the pump control mechanism between the control mechanism and the pump outlet so as to present a simplified structure having a minimum of additional arts.

p It is another object of this invention to provide a starting valve adapted to drain off the output of the pump until it has reached substantially full speed, which is responsive to the rate of fluid flow into the pump outlet and is operable by the pressure drop created by the pressure differential across the ends of a tube incorporated in the valve structure.

It is another object of this invention to provide a starting valve as set forth in the preceding objects which is rapid and positive in operation and seals quickly and tightly with a snap action once the pump output has reached the level required to close the valve.

Further features and advantages of this invention will readily become apparent to those skilled in the art upon a more complete understanding of the preferred embodiment thereof as shown in the accompanying drawings and described in the following detailed description.

In the drawings:

FIG. 1 is a longitudinal cross-sectional view through a variable displacement pump incorporating the starting valve of the present invention;

FIG. 2 is an enlarged fragmentary cross-sectional view of the starting valve in the closed position;

FIG. 3 is a fragmentary cross-sectional view similar to FIG. 2, but showing the valve in the open position;

and

FIG. 4 is a fragmentary view of the spring washer construction of the starting valve.

Referring to the drawings in greater detail, the pump as shown in FIG. 1 comprises a housing 10 which encloses a fluid chamber 11 within which the pump mechanism is located. At one end, housing 10 is formed into a cylindrical end portion 12 which is closed off by an end cap 13 seated against an inturned flange 14 on the housing 10. A seal is provided by O-ring 15 between end cap 13 and pump 10, and the end cap is held in place by suitable fastening means such as set screws 16. A pump outlet 17 is provided on the end cap 13 and is internally threaded to receive a pipe fitting. The fluid chamber 11 is filled with fluid from an external reservoir through an inlet 18 in the pump housing wall, and it will be understoodthat the chamber 11 is completely filled with fluid at all times during the operation of the pump.

At the other end, the housing 10 is closed off by a cover plate 20 which is held in place by cap screws 21. The cover plate 20 is provided with a flange 22 for mounting the pump on an electric motor or other suitable power source. An axial bore 24 extends centrally through the cover plate 20 and supports a bearing member 25 having a radially extending portion 26 adjacent the inner wall of the cover plate 20. A drive member 28 is rotatably journaled on the bearing member 25 and extends axially outward through bore 24 where it is provided with a keyed end portion 30 for receiving a pulley, gear or other suitable driving means. An oil seal 29 is provided at the outer end of bore 24- to prevent leakage of fluid from the pump housing. The bearing member 25 has a radial face 31 adjacent to the radial portion 26 of bearing member 25 to provide a suitable thrust bearing against the pumping loads as described in greater detail hereinafter. The drive member 28 also has an oblique face 32 from which a hub portion 33 extends normal to the oblique face 32. A second bearing member 35 is mounted on hub portion 33 and has a radial portion 36 extending along the oblique face 32.

A transverse web member 38 is mounted within the pump housing 10 intermediate the end cap and cover plate and is fixedly secured in place by a shoulder 37 in the pump housing and a snap ring 39. An extension 40 projects axially from web member 38 toward the drive member 28 and carries a bearing bushing 41 to receive the pilot end 42 of the drive member 28. A plurality of passages or holes 43 extend through the Walls of extension 40 to permit free communication of fluid within the chamber 11 on both sides of the web member 38.

A wobble plate 44 is rotatably journaled on the second bearing member 35 on 'the drive member 28 and abuts against the radial portion 36 to take the thrust loads of the pumping pistons. On its lower side, the wobble plate 44 carries a projecting stud 4-5 on which is rotatably journaled a bearing block 46. Bearing block 46 fits within a channel-like guide member 47 carried in the pump housing, and thus serves to prevent rotation of the wobble plate 44 about the axis of the drive member 28, yet perrnits the Wobble plate 44- to remain in alignment with oblique face 32 on the drive member through rotation of the stud 45 within the bearing block 46.

A cylinder block is slidably mounted within the Cylindrical end portion 12 of the pump housing 10 for reciprocating movement between the end cap 13 and web member 38. A passage 51 extends through the cylinder block 50 to permit circulation of fluid between both sides of the cylinder block to allow free sliding movement within the pump housing. The cylinder block 50 has an axial groove 53 along its outer periphery to receive the end of a guide pin 55 mounted in the walls of the pump housing 10. Groove 53 and pin 55 allow free axial movement of the cylinder block 50, but prevent relative rotation between the cylinder block and the pump housing to maintain proper alignment between the cylinder block and the wobble plate 44.

A plurality of parallel cylinder bores 56 of uniform diameter extend axially through the cylinder block 50 from end to end. Although only one cylinder bore is shown and described, it will be understood that all of these bores are identical and are spaced uniformly about the cylinder block at the same radius from the central axis of the drive member 28. Intake ports to permit the fluid within the fluid chamber 11 to enter the cylinder bores '56 are provided by an outer slot or port 57 formed on the outer periphery of the cylinder block and opening into the cylinder bores from the outside, and by an inner slot or port 58 connected to the fluid chamber through an enlarged bore 60 located axially in the end of the cylinder block towards the web member 38. Thus, fluid is free to flow through both of the slots or ports 57 and 53 into the cylinder bores 56 at all times when these slots are uncovered by the pistons within the bores.

A piston 62 is slidably fitted within the end of cylinder bore 56 adjacent the web member 38 and has a fiat head portion 63 on the end adjacent the inlet ports 57 and 58. The piston has a tubular skirt portion 65 which extends through an opening 64 in the web member 33 and toward the wobble plate 44. A helical compression spring 66 is fitted around the skirt portion 65 to abut at the one end against web member 38 and at the other end against a spring retainer 67 secured on the end of the piston skirt 65. A piston rod 68 is fitted within tubular skirt portion 65 and at the one end has a ball portion 69 which fits within a cup-like recess 70 formed on the face of the wobble plate 44. At its other end, piston rod 68 has another ball portion 71 which engages a socket on the underside of the head portion 63 of the piston. Thus it will be seen that rotation of the drive member 28 causes an oscillation of the wobble plate 44 to progressively reciprocate the pistons within their respective cylinder bores with a sinusoidal motion.

A tubular reaction piston 73 having an internal bore 74 is fitted within the other end of the cylinder bore 56 and extends toward the end cap 13. A compression spring 75 is fitted around the end of reaction piston 73 and abuts at one end against a spring retainer 76 secured on the end of the reaction piston. The other end of spring 75 abuts against a retainer plate 78 which is secured to the end cap 13 by suitable screws 7 9. The end cap 13 is provided Xvith a bore or chamber 81 in axial alignment with the reaction piston 73 and cylinder bore 56. At its outer end, chamber 81 is closed off by a pipe plug 82 while in its other end there is fitted a port member 84 having an end portion 85 extending outward to make sealing contact with the end of the adjacent reaction piston 73. A cage 87 is also mounted within chamber 81 to support a compression spring 88 which biases a check valve plate 89 against the end of the port member 84. A passage 91 connects chamber 81 to the outlet chamber 17. The tubular reaction piston 73 makes a fluid tight seal with the projecting end 85 of the port member and conducts the fluid within the cylinder bores into the port member, past the check valve plate 89 and through the passage 91 to the outlet. Since the reaction piston and port member make a face to face sealing contact, the tubular reaction piston 73 need not be in precise alignment with the port member 84 and therefore the manufacturing tolerances are not critical as to the relative axial alignment of these parts.

The pump is provided with a control assembly for automatically shifting the cylinder block axially in response to changes in the outlet pressure so as to vary the position of the cylinder inlet ports 57 and 58 relative to the stroke of the pistons. As the cylinder block is moved toward the end cap 13, the inlet ports are covered only at successively later portions of the piston stroke, so that a greater portion of the oil within the cylinder bores is forced outward through the inlet ports and a lesser portion is pumped out past the check valve plate 89 after the piston has closed off the port. This control assembly includes a valve body 95 which is fitted within a bore 96 in the cylinder block and is held in place by cooperating flange 97 on the valve body and a snap ring 98. The control valve body 95 extends axially toward the Web member 38 and passes through the enlarged bore 60 in the cylinder block adjacent the inner inlet port 58. The end of the valve body 95 is received within a cylinder member 100 which is secured to the web member 38 by means of neck 101 and snap ring 102.

The control valve body 95 is also provided with a reduced axial bore 104 in which is fitted an axially slida-ble pilot valve spool 105. Valve spool 105 extends into a chamber 106 within the valve body 95 and is there provided with suitable abutment means 107 to receive one end of a helical compression spring 108. The other end of spring 108 fits against an abutment plug 109 secured in the end of chamber 106 by a snap ring 111. Plug 109 is provided with a bore 110 to permit fluid communication between chamber 106 and the chamber 112 within the cylinder member 100. The pilot valve spool 105 has a valving land 114 which cooperates with an annular port 115 in the control valve body. The relative position of the land 114 with respect to the annular port 115 either permits fluid supplied to the bore 104 in the manner to be described hereinafter to be admitted into the control chambers 106 and piston chamber 112 through passage 116, or drains the fluid from these chambers through a drain passage 117 to the fluid chamber 11 of the pump housing.

On the side adjacent the end cap 13, cylinder block 50 is provided with an axially positioned counterbore 120 into which projects a reduced cylindrical portion 121 of the control valve body 95. A cage member 123 is fitted within counterbore 120 and has cylindrical side walls 124 which fit around the cylindrical portion 121 to position the cage member against radial movement. A helical compression spring 125 is fitted between the cylinder block 50 and the end cap 13, where it is positioned around a projecting boss 126. The other end of spring 125 abuts against a radial flange 127 formed on cage member 123. Compression spring 125 operates to bias the cylinder block 50 toward the web member 38 so that the cylinder block tends to assume the maximum displacement position. A plug 129 is fitted within end cap 13 between the pump outlet 17 and the fluid chamber 11. Plug 129 is held in place by a radial flange 130 in the outlet chamber and a snap ring 131. The plug is provided with a suitable O-ring seal 132 to prevent leakage between the high fluid pressure in the outlet chamber and a low pressure within the pump housing.

The plug 129 has an axially disposed bore 133 in which a hollow cylindrical transfer tube 135 is slidably journaled. The transfer tube has a hollow bore 136 therein, and the end of this bore adjacent the outlet has a reduced end portion 137 providing an orifice or flow restriction for operation of the starting valve as will be described in greater detail hereinafter. Transfer tube 135 extends toward the control valve body 95 through an opening 138 in cage member 123. The end of the transfer tube has a reduced diameter portion 139 which makes abutting contact with the face of a seat member 140 positioned within a counterbore 141 at the outer end of valve bore 104. The reduced diameter of the end portion 139 decreases the area of contact with the seat member 140 thus providing a high unit contact pressure for sealing purposes. The seat member 140 in turn has a bore 142 to communicate the fluid at outlet pressure from the transfer tube 135 to the valve spool 105. Suitable flats 144 on the end of the valve spool permit free flow of fluid within the axial bore 104 up to the valving land 114.

A flanged washer 148 is fitted on the end of transfer tube 135 within the cage member 123 and abuts against a snap ring 149 on the transfer tube. Thus a force applied to the washer 148 will shift the end of the transfer tube 135 away from the seat member 140. The control valve body 95 has a second reduced cylindrical portion 151 surrounding the seat member 140, and a part of wave spring washers 153 and 154 separated by a flat spacer washer 155 are fitted around this reduced portion 151. These wave washers have a number of corrugations or convolutions formed into a relatively thin resilient spring material and normally tend to assume a shape having a maximum axial thickness. Thus, the inner wave washer 153 abuts against a shoulder 152 on the control valve 95, while the outer wave washer 154 abuts against the flanged washer 14% which, through the snap ring 149, biases the transfer tube 135 away from the seat member 140 to form a gap therebetween. Movement of the transfer tube 135 is limited by the engagement of the flanged washer 148 with the inside of cage member 123.

In this position, which occurs when the pump is started and there is substantially no pressure within the outlet 17, the fluid which is pumped into the outlet 17 will pass through the hollow bore 136 in the transfer tube and then outwardly through the gap between the end of the transfer tube and the seat member 140. The convolutions of the Wave washers 153 and 154 allow free flow of the fluid then outward through suitable openings 157 in the side wall 124 of the cage member and into the fluid chamber 11. The rate of flow through the hollow bore 136 increases as the pump speed increases and the pressure within the outlet 17 rises, and this increased flow through the orifice portion 137 creates an increasing pressure differential across this orifice. This pressure differential causes a net pressure force on the transfer tube biasing it toward the seat member 140, and when this force reaches a sufficiently high level as the pump approaches full speed, it exceeds the biasing force of the wave washers 153 and 154, and these washers start to compress. *It is a characteristic of these wave spring Washers, as a result of the high angularity of the sides of the convolutions, that they resist substantial deflection until the force exceeds a predetermined level, after which they collapse andreduce their axial length with little increase in the applied force. Thus when the force of the'pressure differential across orifice portion 137 reaches a level where the wave washers start to collapse, they collapse completely and allow this pressure differential force to bring the reduced end portion .139 of the transfer tube into abutting sealing contact with the seat member 140. Leakage of fluid out of the transfer tube then ceases and the pressure will build up rapidly within the outlet 17 to the pump output level and the force of this outlet pressure acting on the cross-sectional area of the transfer tube 135 will then maintain the sealing engagement between the transfer tube and the seat member 140. Once the gap has been closed and the outlet pressure increases, the pressure within the transfer tube 136 will be transmitted through the seat mem- 6 her bore 142 to the pilot valve spool 105. This pressure on the pilot valve spool as opposed by the compression spring 108 determines the position of the valve land 114 relative to the annular port 115 so as to control the flow of fluid into and out of the chambers 106 and 112.

When the pressure within the outlet 17 and hence acting on the effective area of the pilot valve spool exceeds the biasing force of spring 108 and the force applied to the valve spool by the pressure within chamber 106, the valve spool 105 will be forced into the chamber 106 to compress the spring 108. This permits the outlet fluid pressure within the axial bore 104 to flow into the annular port 115 and hence through passage 116 into the chamber 106. Since chamber 106 communicates through plug bore 110 with piston chamber 112, the pressure within this later chamber acts upon the control valve body 95 as a piston to force the valve body and the cylinder block 50 toward the end cap 13 to reduce the effective output volume of the pump. Likewise, when the pressure within the transfer tube 136 drops below the predetermined level, the valve 105 will move outward toward the transfer tube so that valve laud 114 uncovers annular port 115 to connect the chambers 106 and 112 to the fluid chamber 11 to drain passage 117. In this case the biasing force of the cylinder block spring will force the fluid outward through these passages allowing the cylinder block to shift toward the web member 38 to increase the effective volume of the pump and restore the outlet pressure. Thus the starting valve operates to prevent pressure build up within the outlet 17 only while the pump is starting, and once the pump has reached full operating speed and capacity, this valve will close to allow the outlet pressure to build up and to permit operation of the control valve assembly for shifting the position of the cylinder block 50 in the manner disclosed in greater detail in co-pending application. Serial No. 88,142.

It will be understood that various modifications can be made in the structure of the starting valve such as by replacing the wave washers with a coil spring, although wave washers are the preferred form since they provide a spring having a relatively high axial force and rapid collapse in a small area while providing a minimum of interference with the fluid flow radially past these springs. Various other modifications and rearrangements may be resorted by those skilled in'the art without departing from the scope of the invention as defined in the following claims.

What is claimed is:

1. A variable volume pump comprising a pump housing, a fluid chamber within said pump housing, an end cap at one end of said pump housing, an outlet chamber in said end cap, a cylinder block in said pump housing, a plurality of cylinder bores in said cylinder block, a piston within each of said cylinder bores, drive means for reciprocating said pistons, means connecting said cylinder bores to said outlet chamber, control means in said cylinder block for varying the effective pumping stroke of said pistons in response to the pressure in said outlet chamber, a transfer tube slidably journaled in said end cap and making sealing engagement with said control means to interconnect said outlet chamber and said control means, said transfer tube being shiftable out of sealing engagement with said control means to interconnect said outlet chamber and said fluid chamber, means biasing said transfer tube out of sealing engagement with said control means, and flow responsive means constructed and arranged to shift said transfer tube to make sealing engagement with said control means responsive to the rate of fluid flow through said transfer tube.

2. A variable volume pump comprising a pump housing, a fluid chamber within said pump housing, an end cap at one end of said pump housing, an outlet chamber in said end cap, a cylinder block mounted for axially slidable movement in said pump housing, a plurality of cylinder bores in said cylinder block, a piston within each of said cylinder bores, drive means for reciprocating said pistons, means connecting said cylinder bores to said outlet chamber, control means in said cylinder block adapted to shift said cylinder block axially to vary the effective pumping stroke of said pistons in response to the pressure in said outlet chamber, a transfer tube slidably journaled in said end cap interconnecting said outlet chamber and said control means, said transfer tube being shiftable away from said control means to interconnect said transfer tube and said fluid chamber, means biasing said transfer tube away from said control means, and flow responsive means constructed and arranged to shift said transfer tube to make sealing engagement with said control means responsive to the rate of fluid flow through said transfer tube.

3. A variable volume pump comprising a pump housing, a fluid chamber within said pump housing, an end cap at one end of said pump housing, an outlet chamber in said end cap, a cylinder block mounted for axially slidable movement in said pump housing, a plurality of cylinder bores in said cylinder block, a pistonwithin each of said cylinder bores, drive means for reciprocating said pistons, means connecting said cylinder bores to said outlet chamber, control means in said cylinder block adapted to shift said cylinder block axially to vary the effective pumping stroke of said pistons in response to the pressure in said outlet chamber, a transfer tube slidably journaled in said end cap interconnecting said outlet chamber and said control means, said transfer tube being shiftable away from said control means to interconnect said transfer tube and said fluid chamber, means biasing said transfer tube away from said control means, stop means on said cylinder block to limit movement of said transfer tube away from said control means, and flow responsive means constructed and arranged to shift said transfer tube to make sealing engagement with said control means responsive to the rate of fluid flow through said transfer tube.

4. A variable volume pump comprising an elongated generally cylindrical housing, an end cap at one end of said housing having an outlet chamber therein, a cylinder block mounted within the housing for axial movement to a position adjacent said outlet chamber for minimum output position and spring means to bias said cylinder block to a position remote from said outlet chamber for maximum output position, said cylinder block having a plurality of cylinders therein, a reciprocating piston in each of said cylinders, rotating cam means to reciprocate said pistons and bearing means for said rotating cam means, means to conduct fluid from said outlet chamber to the interior of said housing for relieving the load on said bearings at the starting of pump operation, said means comprising a tube carried by said cylinder block and extending axially therefrom into said outlet chamber, valve means constructed and arranged with said tube and spring means normally biasing said valve to open position in response to low pressure in said outlet chamber.

5. A variable volume pump comprising an elongated generally cylindrical housing, an end cap at one end of said housing having an outlet chamber therein, a cylinder block mounted within the housing for axial movement to a position adjacent said outlet chamber for minimum output position and spring means to bias said cylinder block to a position remote from said outlet chamber for maximum output position, said cylinder block having a plurality of cylinders therein, a reciprocating piston in each of said cylinders, rotating drive means to reciprocate said pistons and bearing means for said rotating drive means, means to conduct fluid from said outlet chamber to the interior of said housing for relieving the load on said bearings at the starting of pump operation, said means comprising a tube carried by said cylinder block and extending axially therefrom into said outlet chamber, the fluid pressure in said outlet biasing the tube in one direction, spring means biasing said tube in an opposite direc tion to open position to conduct fluid to the interior of the housing in response to low pressure in said outlet chamber.

6. A variable volume pump comprising an elongated generally cylindrical housing, an end cap at one end of said housing having an outlet chamber therein, a cylinder block mounted within said housing for axial movement to a position adjacent said outlet chamber for minimum r output position, spring means biasing said cylinder block to a position remote from said outlet chamber for maximum output position, control means in said cylinder block adapted to shift said cylinder block axially against the bias of said spring means to said minimum output position, said cylinder block having a plurality of cylinders therein, a reciprocating piston in each of said cylinders, rotating cam means to reciprocate said pistons and bearing means for said rotating cam means, means to conduct fluid from said cylinders to said outlet chamber, means to conduct fluid from said outlet chamber to the interior of said housing for relieving the load on said bearings at the starting of pump operation, said last mentioned means comprising a tube carried by said cylinder block and extending axially from said control means into said outlet chamber, valve means for said tube, and spring means normally biasing said valve to open position in response to low pressure in said outlet chamber. 7. A variable volume pump comprising an elongated generally cylindrical housing, an end cap at one end of said housing having an outlet chamber therein, a cylinder block mounted within said housing for axial movement to a position adjacent said outlet chamber for minimum output position, spring means to bias said cylinder block to a position remote from said outlet chamber from maximum output position, control means in said cylinder block adapted to shift said cylinder block against the bias of said spring means to the minimum output position, said cylinder block having a plurality of cylinders therein, a reciprocating piston in each of said cylinders, rotating drive means to reciprocate said pistons and bearing means for said rotating drive means, means to conduct fluid from said cylinders to said outlet chamber, means to conduct fluid from said outlet chamber to the interior of said housing for relieving the load on said hearings on the starting of pump operation, said last mentioned means comprising a tube carried by said cylinder block and extending axially therefrom into said outlet chamber, the fluid pressure in said outlet biasing the tube in one direction, spring means carried on said cylinder block biasing said tube in the opposite direction to open position to conduct fluid to the interior of the housing in response to low pressure in said outlet chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,512,799 Huber June 27, 1950 2,562,615 Huber July 31, 1951 2,664,048 Huber Dec. 29, 1953 2,990,781 Tuck et al July 4, 1961 FOREIGN PATENTS 402,603 Great Britain Dec. 7, 1933 1,202,109 France July 20, 1959 

1. A VARIABLE VOLUME PUMP COMPRISING A PUMP HOUSING, A FLUID CHAMBER WITHIN SAID PUMP HOUSING, AN END CAP AT ONE END OF SAID PUMP HOUSING, AN OUTLET CHAMBER IN SAID END CAP, A CYLINDER BLOCK IN SAID PUMP HOUSING, A PLURALITY OF CYLINDER BORES IN SAID CYLINDER BLOCK, A PISTON WITHIN EACH OF SAID CYLINDER BORES, DRIVE MEANS FOR RECIPROCATING SAID PISTONS, MEANS CONNECTING SAID CYLINDER BORES TO SAID OUTLET CHAMBER, CONTROL MEANS IN SAID CYLINDER BLOCK FOR VARYING THE EFFECTIVE PUMPING STROKE OF SAID PISTONS IN RESPONSE TO THE PRESSURE IN SAID OUTLET CHAMBER, A TRANSFER TUBE SLIDABLY JOURNALED IN SAID END CAP AND MAKING SEALING ENGAGEMENT WITH SAID CONTROL MEANS TO INTERCONNECT SAID OUTLET CHAMBER AND SAID CONTROL MEANS, SAID TRANSFER TUBE BEING SHIFTABLE OUT OF SEALING ENGAGEMENT WITH SAID CONTROL MEANS TO INTERCONNECT SAID OUTLET CHAMBER AND SAID FLUID CHAMBER, MEANS BIASING SAID TRANSFER TUBE OUT OF SEALING ENGAGEMENT WITH SAID CONTROL MEANS, AND FLOW RESPONSIVE MEANS CONSTRUCTED AND ARRANGED TO SHIFT SAID TRANSFER TUBE TO MAKE SEALING ENGAGEMENT WITH SAID CONTROL MEANS RESPONSIVE TO THE RATE OF FLUID FLOW THROUGH SAID TRANSFER TUBE. 